The behaviour of malaria vectors in huts sprayed with DDT and with a mixture of DDT and malathion in Northern Nigeria

Abstract

1. 1) Because of the failure to interrupt malaria transmission in Northern Nigeria with a twice-yearly house-spraying with 2.0 g./m. 2 DDT water-dispersible powder, and because it was known that the addition of small quantities of malathion to DDT increased mosquito mortalities, field trials were initiated to investigate the effectiveness of DDT, and a mixture of DDT and malathion. 2. 2) Four huts were sprayed with 2.0 g./m. 2 DDT water-dispersible powder and another four with 2.0 g./m. 2 DDT and 0.5 g./m. 2 malathion water-dispersible powder, unsprayed huts acted as controls. 3. 3) A long-lasting repellant effect was observed in both types of sprayed huts, and caused a reduction in the number of A. gambiae and A. funestus entering them. 4. 4) There was a notable reduction in the DDT and DDT/malathion sprayed huts in the proportion of mosquitoes resting in them, and an increase in the numbers leaving. 5. 5) In the DDT huts the irritant effect of the insecticide prevented a considerable proportion of A. gambiae from feeding, and this resulted in a pronounced increase in the number of unfeds found in, and leaving, these huts. 6. 6) In the DDT/malathion huts there was no marked reduction in the biting rate of A. gambiae and only a slight increase in the number of unfed A. funestus, but both species left the sprayed huts before completion of the gonotrophic cycle. 7. 7) Both insecticidal formulations produced a small, but appreciable, overnight room kill. 8. 8) A 70 per cent. mortality in A. gambiae was achieved only during the first 4 months in the DDT huts, after this unsatisfactory kills were obtained. 9. 9) In the DDT/malathion huts mortalities were 70 per cent. or above for both species for 5 months, and although that of A. funestus dropped to 47 per cent. in the last 2 months, a 60 per cent. kill was recorded for A. gambiae in the sixth month. 10. 10) Mortalities of the blood-fed fractions were high for 6 months in the DDT/malathion huts, but for only 4 months in the DDT huts. 11. 11) It is concluded that the addition of a small amount of malathion to DDT increases mortality, improves the effective persistence of the insecticide, and causes a reduction in irritability.